Adaptive responses to oxygen stress in obligatory anaerobes Clostridium acetobutylicum and Clostridium aminovalericum.

Clostridium acetobutylicum and Clostridium aminovalericum, both obligatory anaerobes, grow normally after growth conditions are changed from anoxic to microoxic, where the cells consume oxygen proficiently. In C. aminovalericum, a gene encoding a previously characterized H2O-forming NADH oxidase, designated noxA, was cloned and sequenced. The expression of noxA was strongly upregulated within 10 min after the growth conditions were altered to a microoxic state, indicating that C. aminovalericum NoxA is involved in oxygen metabolism. In C. acetobutylicum, genes suggested to be involved in oxygen metabolism and genes for reactive oxygen species (ROS) scavenging were chosen from the genome database. Although no clear orthologue of C. aminovalericum NoxA was found, Northern blot analysis identified many O2-responsive genes (e.g., a gene cluster [CAC2448 to CAC2452] encoding an NADH rubredoxin oxidoreductase-A-type flavoprotein-desulfoferrodoxin homologue-MerR family-like protein-flavodoxin, an operon [CAC1547 to CAC1549] encoding a thioredoxin-thioredoxin reductase-glutathione peroxidase-like protein, an operon [CAC1570 and CAC1571] encoding two glutathione peroxidase-like proteins, and genes encoding thiol peroxidase, bacterioferritin comigratory proteins, and superoxide dismutase) whose expression was quickly and synchronously upregulated within 10 min after flushing with 5% O2. The corresponding enzyme activities, such as NAD(P)H-dependent peroxide (H2O2 and alkyl hydroperoxides) reductase, were highly induced, indicating that microoxic growth of C. acetobutylicum is associated with the expression of a number of genes for oxygen metabolism and ROS scavenging.